I've noticed a trend (at least on the Google Play store) of application prices ending in x.99, while 'donation' apps usually end on the dollar. I see the reasoning behind the psychology that makes a user purchase a $0.99 item over one that costs $1.00, but why the trend to keep the prices on the dollar for donation items?
The psychology behind the $0.99 was explored in depth in Priceless: The Hidden Psychology of Value, which if you ask for my humble opinion, is a life-changing book. Partly the reason for such price tags is that it translates for many as a 'sale' price. Against it, is that it is typically associated with 'hard sale'.
The donation payment system is in its core anti-capitalist, thus anti-commerce, and so prices are accordingly just simple (distinguishing themselves from any sale or hard sale like prices).